In the oil and gas industry, there is a need for efficient testing of pipelines. Such pipelines are often difficult to access, e.g. offshore pipelines that may be partly buried, and which forms extended structures. The pipelines are subject to wear from corrosive fluids and sand, and deformation from movements in the seabed. Said pipelines are also prone to developing cracks, in particular in the welding seams. Welding seams are inherent weak points due to the changes in the steel structure caused by the welding process. Cracks may develop due to stress caused by temperature cycling and movements in the seabed.
The structural integrity of pipelines may be tested using inspection pigs which travel inside the pipelines measuring the condition of the pipe wall. There have been devised several methods for measuring the condition of pipeline walls. Here we will mention methods using magnetic flux leakage and ultrasonic testing. Methods using magnetic flux leakage are mainly effective only for detecting metal loss (thinning of the pipeline wall) caused by corrosion. Ultrasonic testing methods are used for detecting corrosion and cracks in pipeline walls, even though there is some overlap between the technologies. Ultrasonic testing using conventional piezoelectric transducers are limited to testing pipelines filled with liquids, as a liquid is needed to conduct the ultrasonic energy into the pipeline wall. The extreme difference in acoustic impedance between air/gas and steel will greatly reduce the amount of acoustic energy being conducted into the pipeline wall in a “dry” pipeline. It has been proposed to use Electro Magnetic Acoustic Transducer (EMAT) technology for testing gas pipelines; this type of transducer generates an electromagnetic field which introduces Shear Horizontal (SH) wave mode ultrasonic waves directly into the pipeline wall. However, such transducers are large, have a limited bandwidth, and must be positioned very close to the pipe wall, preferably less than 1 mm from the wall surface.
From U.S. Pat. No. 8,677,823 there is known a setup wherein a spool carrying acoustical transducers (in an array around the central narrow part of the spool) is sent through a pipeline containing pressurised gas. Acoustic signals are transmitted from each transducer, through the gas medium to the inner surface of the pipe wall.
These are reflected back by the wall and received by the same transducer or by a preselected transducer(s) in the spool. This equipment is used for measuring the diameter of the pipeline to identify deformations to the wall. However, this setup is not suited for testing the material in the wall itself, due to the high impedance contrast between air and steel.
European patent application EP 2 887 060 A1 discloses an apparatus for inspecting pipelines. This application was filed on 20 Dec. 2013, and published on 24 Jun. 2015. The pipe wall is inspected by means of acoustical pulses emitted from an array of transducers, the transducers being localized in a single ring or row around the body of the apparatus, see FIG. 1. The measurements are pulse-echo-measurements, wherein pulses are transmitted and received by the same transducer.
From U.S. Pat. No. 9,852,033 there is known an apparatus for logging oil and gas wells. The apparatus includes a rotating transducer head with three acoustic transducers. The transducer head is rotated while the apparatus is displaced vertically along the well. In this way, the well may be covered by a series of individual measurements covering the wall along a spiral path.